My Husband Has Been On Seroquel For Well Over 20

my husband has been on Seroquel for well over 20 years. 2+ years ago, he began having cognitive impairment issues and was recently diagnosed with Frontal Lobe Dementia. He takes 800 mg. Seroquel every night which does nothing for his sleep and now his neurologist wants to explore whether the "dementia" symptoms he has might be related to his taking this high dosage of Seroquel for so many years. Is this possible?